This invention relates to fuel cell power plants and more particularly, to fuel cell power plants utilizing a water transport plate having interdigitated flow channels therein to furnish the reactant gases to the fuel cell.
Fuel cell power plants are electrochemical alternative power sources for both stationary and mobile applications. The fuel cell, which is the heart of such power plant, consists of an anode, a cathode and an electrolyte that separates the two. Anode shall mean a negative electrode, and cathode shall mean a positive electrode. In the operation of a fuel cell, fuel reactant gas, which is typically a hydrogen rich stream, enters a support plate that is adjacent to the anode. Such a support plate is, therefore, referred to as an anode support plate. Also, oxidant reactant gas, which is commonly air, enters a support plate adjacent to the cathode. This support plate is, therefore, referred to as a cathode support plate. As the hydrogen rich stream passes through the anode support plate, a catalyst located between anode support plate and the electrolyte causes the hydrogen to oxidize, thereby resulting in the creation of hydrogen ions and electrons. While the hydrogen ions migrate through the electrolyte to the cathode, the electrons migrate through an external electrical circuit to the cathode. Another catalyst on the cathode side of the electrolyte causes the oxygen to react with the hydrogen ions and electrons released at the anode, thereby forming water. The occurrence of these reactions near the catalysts and electrolyte creates an electric potential across the fuel cell. The flow of electrons through an external circuit that is connected to the fuel cell produces useful work, such as powering an electric motor in a vehicle.
There are various types of fuel cells, which vary according to their electrolyte. The electrolyte is the ionic conducting substance between the anode and the cathode. One type of fuel cell includes a solid polymer electrolyte or otherwise referred to as a proton exchange membrane (PEM). Fuel cells incorporating a solid polymer electrolyte (proton exchange membrane) will hereinafter be referred to as a PEM fuel cell. The catalyst layers within a PEM fuel cell are typically attached to both sides of the membrane, thereby forming a membrane electrode assembly (MEA). As noted above, while hydrogen ions pass through the MEA, the electrochemical reaction between the hydrogen ions, electrons, and oxidant reactant gas forms water within the cathode. This water is commonly referred to as xe2x80x9cproduct water.xe2x80x9d In addition, water may also accumulate in the cathode, due to the drag of water molecules, which pass from the anode and through the MEA along with the hydrogen ions during the operation of the fuel cell. This water is commonly referred to as xe2x80x9cproton drag water.xe2x80x9d The proton drag from the anode to the cathode results in a lower water content on the anode side of the PEM compared to the cathode side. This difference in water content between the anode and cathode sides results in an osmotic force, which fosters the flow of water from the cathode side of the PEM towards the anode side. However, if the PEM (i.e., electrolyte) doesn""t remain highly saturated with water, the PEM resistance increases, and the useful power obtained from the fuel cell decreases. Additionally, if product water and drag water accumulate in the cathode, the accumulated water may impede and could prevent oxygen from reacting with the hydrogen ions and electrons. Accumulation of water in the cathode will thus reduce the electric potential created across the fuel cell, thereby limiting the fuel cell""s performance. Furthermore, if the cathode water content fails to decrease, the cathode will flood, and the fuel cell will eventually cease to produce power and shut down.
In order to assist the oxidant reactant gas in reaching the catalyst on the MEA, the cathode support plate typically comprises a diffusion layer and a substrate layer. Both the diffusion layer and the substrate layer are typically constructed of porous carbon layers that are rendered hydrophobic. Hydrophobic means antagonistic to water and is therefore often referred to as wet-proofed. It is known, however, to utilize a hydrophilic substrate in lieu of a hydrophobic substrate. Hydrophilic means capable of absorbing water and therefore, is often referred to as wettable. U.S. Pat. No. 5,641,586, for example, describes a cathode support plate comprising a hydrophilic substrate layer and a hydrophobic diffusion layer. Objects of U.S. Pat. No. 5,641,586 included providing a porous support plate which reduced the pressure drop of the oxidant gas as it passed through such support plate, minimizing water accumulation within such support plate and maximizing access of the oxidant reactant gas to the catalyst. Although a hydrophilic substrate may reduce the pressure drop of the oxidant reactant gas through the cathode, the hydrophilic substrate, by its inherent nature, absorbs more water than a hydrophobic substrate. Therefore, unless the water is properly removed from the cathode support plate, the hydrophilic substrate will absorb the water, which, in turn, will eventually flood the cathode support plate. Flooding the cathode support plate would, therefore, negate one of the objects of U.S. Pat. No. 5,641,586: namely, the object relating to minimizing water accumulation.
Flooding the cathode support plate would also prevent the oxidant reactant gas from reaching the catalyst. U.S. Pat. No. 5,641,586 describes a cell operating at elevated pressure, and product water within such a cell typically exits the cell via the oxidant reactant gas exhaust stream as a combination of water vapor and entrained liquid water. Entrained liquid water moves along a reactant flow channel from the interior of the cell to the oxidant reactant gas exhaust stream. This concept is well accepted for cell configurations which utilize a hydrophobic substrate and a solid reactant support plate. U.S. Pat. No. 5,641,586, however, describes a cell with a hydrophilic substrate, which will absorb the liquid water and flood the substrate, thereby impeding transport of oxygen to the cathode catalyst.
U.S. Pat. No. 5,641,586 also describes a diffusion layer or PEM layer adjacent to one side of the hydrophilic substrate and a solid flow-field plate adjacent to the other side of the hydrophilic substrate. The solid flow-field plate, which is also referred to as a separator plate, defines the flow channels for the reactant gases to pass through. Because the flow-field plate is solid (i.e., non-porous) and impermeable to gas and liquid, the channels not only supply the fuel cell with oxidant reactant gas but may also serve as exit passageways for the product water. However, in order to force the product water through the channels, the pressure of the oxidant reactant gas stream must be relatively high (five atmospheres in said patent), which is an undesirable operating condition.
Objects of the invention include a fuel cell power plant that efficiently and properly removes the product and proton drag water from the cathode, thereby ensuring that the maximum amount of oxygen from the oxidant reactant gas stream reaches and reacts with the catalyst on the cathode side of the MEA.
The present invention is a fuel cell power plant that includes a fuel cell having a membrane electrode assembly (MEA), which is disposed between an anode support plate and a cathode support plate and wherein the anode and/or cathode support plates include a hydrophilic substrate layer. The fuel cell power plant also includes a fuel reactant gas stream, which is in fluid communication with the anode support plate""s hydrophilic substrate layer, and an oxidant reactant gas stream, which is in fluid communication with the cathode support plate""s hydrophilic substrate layer, and a cooling water stream, which is in fluid communication with both the anode and cathode support plate hydrophilic substrate layers. The hydrophilic substrate layer in the anode support plate enhances the migration of the cooling water to the anode side of the MEA, and the hydrophilic substrate layer in the cathode support plate improves the removal of water from the cathode side of the MEA. The hydrophilic substrate layers within both the anode and cathode support plates have a predetermined level of porosity (i.e., number of pores) and pore size. The inventors of the present invention recognized that without controlling the number of pores within the hydrophilic substrate that contain water, the water will fill 100% of the available pores, thereby preventing any migration of the reactant gases. The inventors of the present invention, therefore, recognized that controlling the pressure of the reactant gas streams and the coolant stream, controls the percentage of pores that contains water or reactant gas. The present invention utilizes a pressure differential between the coolant stream and the reactant gas streams to control the respective distribution of the streams within the pores of a hydrophilic substrate. The pressure differential is established such that a greater percentage of the pores within the hydrophilic substrate layer contain reactant gas rather than water. The present invention, therefore, provides a means for creating a pressure differential between the reactant gas streams and the coolant stream such that the pressure of the reactant gas streams is greater than the pressure of the coolant stream. Operating the fuel cell at such pressure differential ensures that the product and proton drag water that form at the cathode catalyst layer will migrate through the cathode support plate and away from the MEA. Controlling the coolant and fuel reactant gas streams on the anode side of the MEA also ensures that the cooling water will continually migrate from the coolant stream toward the anode side of the MEA, thereby preventing the membrane from becoming dry.
Proper water balance in the cathode and anode support plates ensures that the PEM remains moist, thereby prolonging the fuel cell""s life, as well as improving its electrical efficiency. Proper water removal also facilitates increased oxygen utilization within the fuel cell. Specifically, without proper water removal, a reduced portion of the available oxygen reaches the catalyst. Increasing the amount of oxygen that is available at the catalyst increases the fuel cell""s performance and/or reduces the overall size of the fuel cell in order to generate a certain power rating. Although U.S. Pat. No. 5,641,586 recognized that replacing the cathode hydrophobic substrate layer with a hydrophilic substrate layer increased the cell voltage, that patent failed to discuss the importance of water removal from the cathode. Moreover, that patent failed to teach that controlling the pressure of the reactant gas streams and the coolant streams controls the filled porosity of the hydrophilic substrate, as well as reducing the water content of the catalyst layers. The present invention, in contrast, provides a means for preventing the cathode support plate from flooding, thereby ensuring that the maximum amount of oxidant reactant gas reaches the cathode side of the MEA. Hence the present invention not only improves the electrical power output capacity of a fuel cell but also increases the fuel cell""s oxygen utilization, which, in turn, further improves the fuel cell system""s operational efficiency.
Accordingly, the present invention relates to a fuel cell power plant comprising a fuel cell, which includes an anode support plate and a cathode support plate and a membrane electrode assembly disposed between the anode and cathode support plates, wherein the membrane electrode assembly includes a polymer electrolyte membrane disposed between two catalysts, and wherein the anode support plate and the cathode support plate each contain a porous hydrophilic substrate layer. The fuel cell power plant also includes porous water transport plates adjacent to the anode and cathode support plates, thereby enhancing the fuel cell""s ability to remove water from the cathode support plate and to transfer water through the anode support plate to the membrane. Specifically, the water transport plates have passageways on one side that allow the coolant stream to pass therethrough and separate passageways on its other side that allow the reactant gas streams to pass therethrough. Because the cathode water transport plate is porous, the pores allow the product water to flow from the substrate layer into and through the cathode water transport plate and eventually into the coolant stream. Hence, water escapes from the fuel cell via the coolant stream rather than the reactant passageways, as in U.S. Pat. No. 5,641,586. As a result, the fuel cell operates at a lower pressure. Likewise, the pores within the anode water transport plate allow the water to flow from its coolant passageways to the anode support plate. In order to facilitate an efficient flow of water, the fuel cell power plant further includes a means for creating a predetermined pressure differential between the reactant gas streams and the coolant stream such that the pressure of the reactant gas streams is greater than the pressure of the coolant stream.
In another embodiment of the present invention, the water transport plates include interdigitated passageways (i.e., flow channels) in lieu of conventional passageways for the reactant gas streams to pass therethrough. Interdigitated passageways shall mean at least two distinct passageways separated by a dividing wall (i.e., rib) such that the reactant gas stream enters one distinct passageway (i.e., entry passageway) and exits the other distinct passageway (i.e., exit passageway). In order for the reactant gas stream to migrate from the entry passageway to the exit passageway, the reactant gas stream passes through the anode and cathode support plates, thereby increasing the mass transfer of the reactant gas streams to the anode and cathode support plates. Increasing the quantity of reactant gases that reach the catalysts, in turn, increases the electrical performance of the fuel cell. Hence, utilizing interdigitated flow channels in lieu of conventional flow channels within a porous water transport plate increases the electrical performance of the fuel cell while maintaining an efficient water management system.
In an other embodiment of the present invention, the interdigitated passageways are included within the anode and/or cathode substrate layers in lieu of within the anode and/or cathode water transport plates.
In other embodiments of the present invention, the anode and/or cathode support plates may contain a diffusion layer. If so, it is preferable that the diffusion layer be partially hydrophobic rather than totally hydrophobic because a partially hydrophobic diffusion layer is capable of transferring a larger percentage of liquid water than a totally hydrophobic diffusion layer, such as described in U.S. Pat. No. 5,641,586.
In a further embodiment of the present invention, a fuel cell has an anode and/or cathode support plate that includes a hydrophilic substrate layer but does not include a diffusion layer. Removing the anode diffusion layer or reducing its thickness increases the migration of water from the water transport plate to the MEA, thereby ensuring proper moisturizing of the PEM, particularly at high current densities, which in turn, further improves the electrical efficiency of the fuel cell. Removing the cathode diffusion layer or reducing its thickness reduces the distance through which the oxidant reactant gas must pass before reaching the catalyst, thereby increasing the fuel cell""s oxygen utilization characteristics.
In accordance with the invention, a substantially atmospheric, PEM fuel cell achieves a maximum current density of about 1.6 amps per square centimeter, operating with air stochiometry of 2.50 (250%) or less.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.